Marcion, the Stranger God and the Demiurge
Adolf von Harnack was a scholar of the history of Christian dogma a century ago, and the first to study Marcion thoroughly in modern times. He concluded that it makes little sense for Christians to retain the Old Testament.
In von Harnack's case this was because he regarded Marcion as the most important figure in the early church. I would go a step further and say that there was no Christianity resembling what we know today, that the New Testament never existed before Marcion, and that the original New Testament, the Euangelion and the Apostolikon was an outright rejection of the jealous God Jehovah, his law and all the unnecessary legalistic requirements like circumcision.
Marcion taught that the Archon, the Creator God or God of the Law who created our bodies and the world was of less importance than the unknowable Stranger God, the God who sent Isu Chrestos. It was essential for the Marcionites that Isu Chrestos was a ghost, as the true God was far too pure to take human form. The Marcionites saw Judaism as a very worldly religion, and Isu Chrestos came to replace legalism with mercy and love.
There are a lot of parallels with Gnosticism here. The Gnostics believed that there was creation before the creation of the physical world. Where there Gnostics before Marcion? The main source of Gnostic gospels was the Nag Hammadi library, but this was dated post Constantine. Marcion is not regarded as a Gnostic by most critics. The Gnostics relied heavily on a direct relationship with God, and personal revelations. Marcion differed not even slightly from modern Christians who strictly follow the book. Of course his book was without the Old Testament, and many of the other books in the New Testament which support the idea that Christianity is a new dispensation of Judaism.